This invention relates to the production of Al(OH).sub.3 and more particularly to the precipitation of Al(OH).sub.3 from a Bayer green liquor.
Conventionally, in the recovery of Al(OH).sub.3 in the Bayer process, raw materials such as bauxite are digested in caustic followed by removal of the red mud residue from the resulting green or pregnant liquor prior to precipitation. Seed particles are then introduced into the green liquor in a number of precipitation tanks well agitated to maintain seed and liquor contact in which precipitation is carried out by hydrolysis according to the following reaction: ##STR1##
The hydrolysis reaction proceeds slowly, normally over a period of 20 to 60 hours, the rate being a function of temperature, seed surface area, and concentrations of caustic, alumina, and impurities. The Al(OH).sub.3 is then classified by hydraulic elutriation with the separated product size fraction removed for calcination where the chemically combined water is driven off at high temperatures. Finer particles are recycled as seed. Depleted or spent liquor leaving the precipitation tank is evaporated and recycled to the digester.
While this process has been very successfully carried out through the years to produce high quality Al(OH).sub.3 suitable for subsequent calcination and use in a smelter for the production of metallic aluminum, there are some shortcomings to the process. Because of the slow reaction rate, as mentioned above, a number of precipitation tanks are required to handle the large volume of green liquor. The process also requires a subsequent classification step wherein larger product size particles are recovered for ultimate use in either smelting or chemical application while the smaller seed particles are returned to the precipitation tanks. Alternatively, filtration is employed to effect separation of Al(OH).sub.3 from liquor.
Alternate or modified precipitation methods are known. For example, Cowles U.S. Pat. No. 1,943,786, suspends particles of Al(OH).sub.3 in a precipitation tank by the introduction of liquor into the bottom of the tank. The liquor is also introduced into the top of the tank and then drawn off to be pumped into the bottom of the tank resulting in mixing and essentially uniform concentrations throughout the tanks.
Mejdell et al U.S. Pat. No. 4,049,773, discloses a multiple stage precipitation process in which green liquor is fed counter-current to the direction of feed of the Al(OH).sub.3 crystals through at least two stages. The patentees show turbulent zones in each of the precipitators which may be obtained by using an agitator.